We present the design strategy of shallow-angle grating couplers for vertical emission from InP devices, and then discuss the focusing effect of a 2D grating. Measured beam shapes from prototyped devices agree well with the simulation results.
NASA is working with US industry and academia to develop Photonic Integrated Circuits (PICs) for: (1) Sensors (2) Analog RF applications (3) Computing and free space communications. The PICs provide reduced size, weight, and power that is critical for space-based systems. We describe recent breakthrough 3D monolithic integration of photonic structures, particularly high-speed graphene-silicon devices on CMOS electronics to create CMOS-compatible highbandwidth transceivers for ultra-low power Terabit-scale optical communications. An integrated graphene electro-optic modulator has been demonstrated with a bandwidth of 30 GHz. Graphene microring modulators are especially attractive for dense wavelength division multiplexed (DWDM) systems. For space-based optical communication and ranging we have demonstrated generating a variable number of channels from a single laser using breadboard components, using a single-sideband carrier-suppressed (SSBCS) modulator driven by an externally-supplied RF tone (arbitrary RF frequency), a tunable optical bandpass filter, and an optical amplifier which are placed in a loop. We developed a Return--to-Zero (RZ) Differential Phase Shift Keying (DPSK) laser transmitter PIC using an InP technology platform that includes a tunable laser, a Semiconductor Optical Amplifier (SOA), high-speed Mach-Zehnder Modulator (MZM), and an electroabsorption (EAM) modulator. A Silicon Nitride (SiN) platform integrated photonic circuit suitable for a spectrally pure chip-scale tunable opto-electronic RF oscillator (OEO) that can operate as a flywheel in high precision optical clock modules, as well as radio astronomy, spectroscopy, and local oscillator in radar and communications systems is needed. We have demonstrated a low noise optical frequency combs generation from a small OEO prototypes containing very low loss (~1 dB) waveguide couplers of various shapes and sizes integrated with an ultrahigh-Q MgF2 resonators. An innovative miniaturized lab-on-a-chip device is being developed to directly monitor astronaut health during missions using ~3 drops of body fluid sample like blood, urine, and potentially other body fluids like saliva, sweat or tears. The first-generation system comprises a miniaturized biosensor based on PICs (including Vertical Cavity Surface Emitting Laser – VCSEL, photodetector and optical filters and biochemical assay that generates a fluorescent optical signal change in response to the target analyte.
A novel 3D hybrid integration platform combines group III-V materials and silicon photonics to yield high-performance lasers is presented. This platform is based on flip-chip bonding and vertical optical coupling integration. In this work, indium phosphide (InP) devices with monolithic vertical total internal reflection turning mirrors were bonded to active silicon photonic circuits containing vertical grating couplers. Greater than 2 mW of optical power was coupled into a silicon waveguide from an InP laser. The InP devices can also be bonded directly to the silicon substrate, providing an efficient path for heat dissipation owing to the higher thermal conductance of silicon compared to InP. Lasers realized with this technique demonstrated a thermal impedance as low as 6.2°C/W, allowing for high efficiency and operation at high temperature. InP reflective semiconductor optical amplifiers were also integrated with 3D hybrid integration to form integrated external cavity lasers. These lasers demonstrated a wavelength tuning range of 30 nm, relative intensity noise lower than -135 dB/Hz and laser linewidth of 1.5 MHz. This platform is promising for integration of InP lasers and photonic integrated circuits on silicon photonics.
We review the recent advancement in the system and device technologies for coherent optical communications. One major topic is high-dimensional modulation, and in particular the nonlinearity-tolerant modulation format family, based on four-dimensional 2A8PSK. This family, covering 5, 6, 7 bits/4D symbol, outperforms most known corresponding modulation formats in the linear and nonlinear region. We also review our recent progress on forward error correction including polar codes, and monolithic narrow linewidth semiconductor lasers.